In recent years, there has been focus on refrigeration cycle apparatuses that uses, as its refrigerant, carbon dioxide (hereinafter, referred to as CO2), which has zero ozone depleting potential and a markedly small global warming potential as compared with those of chlorofluorocarbons. The critical temperature of the CO2 refrigerant is as low as 31.06 degrees C. When a temperature higher than this temperature is used, the refrigerant at a high-pressure side (from the outlet of a compressor, to a radiator, and then to the inlet of a decompressor) of the refrigeration cycle apparatus enters a supercritical state in which no condensation occurs, thereby decreasing operating efficiency (COP) of the refrigeration cycle apparatus as compared with conventional refrigerants. Hence, means for increasing COP is important to refrigeration cycle apparatuses using a CO2 refrigerant.
As such means, there is suggested a refrigeration cycle that is provided with an expander instead of a decompressor and that recovers pressure energy during expansion as power. Meanwhile, in a refrigeration cycle apparatus with a configuration in which a positive displacement compressor and an expander are coupled with a single shaft, when VC is a stroke volume of the compressor and VE is a stroke volume of the expander, a ratio of the volumetric circulation rate of the refrigerants respectively flowing through the compressor and the expander is determined by VC/VE (a design volume ratio). When DC is density of the refrigerant at an outlet of an evaporator (the refrigerant flowing into the compressor) and DE is density of the refrigerant at an outlet of an radiator (the refrigerant flowing into the expander), a relationship of “VC×DC=VE×DE,” that is, a relationship of “VC/VE=DE/DC” is established since the mass circulation rate of the refrigerants respectively flowing through the compressor and the expander are equivalent. VC/VE (the design volume ratio) is a constant that is determined at the time of design of the device. The refrigeration cycle tends to balance itself so that DE/DC (the density ratio) is always constant (hereinafter, this is called “constraint of constant density ratio”).
However, use conditions of the refrigeration cycle apparatus are not necessarily constant, and hence if the design volume ratio expected at the time of the design differs to the density ratio in the actual operating state, it would be difficult to regulate the pressure of the high pressure side to an optimal pressure due to the “constraint of constant density ratio.”
Owing to this, there is suggested a configuration and a control method for regulating the pressure of the high pressure side to the optimal pressure by providing a bypass that bypasses the expander and controlling the amount of refrigerant flowing into the expander (for example, see Patent Literature 1).
Also, there is suggested a configuration and a control method for regulating the pressure of the high pressure side to the optimal pressure by providing a compression bypass that bypasses a phase from a midway position of a compression process of a main compressor to completion of the compression process, by providing a sub-compressor in the compression bypass, and by controlling the amount of refrigerant flowing into the sub-compressor (for example, see Patent Literature 2).